U.S. Pat. No. 4,947,309 discloses a method of providing electric power to a load. Two switch transistors are connected in series between two terminals of the DC power source. The bridge point between the transistors is connected to the load via an inductor. A control circuit is arranged to switch on one of the transistors, for example the transistor connected to the positive terminal. Then, the current through the inductor increases to deliver current to the load. When the current through the inductor exceeds a preset value I0, the transistor is switched off and the current through the inductor takes another way through a flywheel diode of the opposite switch transistor, until the current through the inductor has decreased to zero. At this time instance, the upper transistor is again switched on and a new cycle starts. A corresponding operation takes place at negative currents but using the opposite switch transistor.
Because the switch on of the switch transistor takes place at zero current and also essentially at zero voltages as will be explained below, the switch on power losses of the switch transistor becomes negligible. The switch off consumes approximately the same power as in previously known PWM circuits.
The average current delivered to the load is approximately half of the top value of the current in the inductor. The switch frequency becomes approximately inversely proportional to the preset current value I0. Since the switch losses are proportional to the switch frequency, the efficiency becomes low at low currents.
The switch frequency should be much higher than the frequency of the current delivered to the load. If the load is an electric motor, the motor frequency will normally be less than about 100 Hz. Thus, the PWM frequency should normally be higher than about 5 to 10 kHz. However, modern power transistors of the IGBT type should not be operated at higher frequencies than about 20 kHz. Thus, the operational area of the motor control becomes limited.
Thus, there is a need for a method of operating the motor control system according to U.S. Pat. No. 4,947,309 so that low currents can be controlled without increasing the PWM frequency above a predetermined frequency, for example 20 kHz.
This problem has been addressed in the publication WO 2008/100223, which discloses a switch control circuit comprising a timer circuit, which is arranged for preventing the on-switch of the switch element until a minimum time period has passed. In this way, the frequency is limited to the inverse of said time period.